Method of producing chromised screws



United States Patent 3,118,224 EJTHOD 0F PRGDUCING (ZHRQMISED SCREWS,

NUTS, AND BQLTS Gottfried Becker, Dusseidorf, Germany, assignor to Deutsehe Edelstahiwerke Aktiengesellschaft, Krefeld, Germany No Drawing. Fiied May it), 1960, Ser. No. 27,992. (Iiaims priority, application Gerrnwy May 20, 1959 2 Ciaims. (Cl. 29528) It is known that it is possible to impart to articles made of steel a surface which has a structure similar to that of, and exhibiting the same resistance to corrosion as, a corrosion-resistant steel by difiusing chromium into their surface. The method which is to-day more generally employed consists in heating the treated parts in media which release chromium and to transfer the chromium by means of chromium-halogen compounds in vapour form. The diffusion of the chromium into the surface of the parts gives rise to the formation of a zone which is rich in chromium and has a chromium content of about 50% in the outer layer gradually diminishing to zero with increasing depth. The usual practice is to produce zones which have a chromium-resistant thickness of about 0.1 mm. This is considered to be that part of the chromised zone in which the chromium content is above the threshold of corrosion resistance, in other words, which contains more than 13% chromium, this being the necessary minimum chromium content to render a steel corrosion-resistant. This specification of the depth of the chromised zone is based upon technological considerations which arise in connection with the provision of satisfactory results, the said depth of the zone generally providing sufiicient protection from corrosion, whereas thinner zones are generally not satisfactory.

The conventional method consists in chromising the parts after they have been worked, so that the chromised layer will form a protective zone uniformly covering the whole of the surface. Inter alia screws and bolts are treated in this way after they have been provided with threads, in order to protect them from corrosive attack.

Now, it has been observed that local foci of corrosion do appear on threaded parts, especially on screws, nuts and bolts, in the threaded region. These are due to damage having been done to the chromised zone on these generally more delicate parts of the screw. In practice the trouble cannot be satisfactorily cured by increasing the thickness of the protective chromised layersalthough in principle this could be done by prolonging the period of chromium diffusion or by raising the temperature at which chromising is performed-because the observance of the ecessary tolerances, which already causes trouble when the thickness of the zone is only 0.1 mm., would entail even greater diificulties if the treatment periods were prolonged or the temperatures raised.

Attempts have already been made to depart from the conventional method of producing chromised parts and to roll the threads into the parts after they have been chromised. The chromised zone is capable of paricipating in the deformation of rolling without flaking off, provided a suitable base material has been selected. The base material should preferably contain less than 0.2% carbon. Chromised zones on such steels are known to be mechanically deformable and it is possible to subject chromised threads to a final rolling. This procedure is adopted in order to smooth and consolidate the surface, but it is a special procedure not employed in the normal production of screws and bolts. However, when threads are rolled into parts after they have been chromised, especially if these parts are subsequently exposed to moisture, major foci of corrosion are still observed to occur in the region of the threads. This defect which appears to be due to the nature of the method of forming the threads, in which the edge of the tool which rolls the threads penetrates the surface of the shaft of the bolt and forms the crests by squeezing up the material, militates against the adoption of such a production method and it was in fact abandoned after experiments had been made.

However, despite these initial failures it has now been found that the said method of production is not only feasible but actually affords advantages, provided certain specifiable working rules are observed. According to the present invention a chromised zone must be provided which has a thickness equal to the depth of the subsequently rolled threads divided by 2.5. Furthermore, in view of the displacement of the material into the crests of the threads, care must be taken to ensure that the depth of the zone at the root of the thread is not less than 0.05 mm. The displacement of the material which causes the chromised zone in the crests to be significantly thickened and at the same time to be reduced at the roots has no disadvantageous consequences, if the working rule specified by the invention is strictly adhered to. On the contrary, it has been found that this reduction of the thickness of the zone at the root does not increase sensitivity to corrosive attack, and that rolling the threads after the part has been chromised no only consolidates and improves the mechanical properties of the zone but that the rolled surface surprisingly has a higher overall corrosion resistance provided the protective zone is not too thin and the zone at the bottom of the threads is not less than 0.65 mm. This fact is all the more surprising because it does not apply in the case of zones of like thickness which have not been subjected to rolling.

As utilised hereinafter throughout the specification, the letter M followed by a numeral defines the size of a screw thread, the letter representing that the size is denoted in the metric system and the numeral indicating the major diameter in millimeters. This designat on of screw thread size is a German standard description of screws, bolts, etc. with a metric thread. Examples of German use of this nomenclature may be found in the book entitled Schrauben, Muttern and Zubehor fiir metrische Gewinde, 6th edition, January 1948, published by the German Standards Commission.

Since for technological and economic reasons, the depth of the zones cannot be arbitrarily increased, the proposed method is in practice particularly suitable for bolts and screws of sizes under M12. In this range normal production can be based on the simple relationship which is expressed by the formula Z:t:2.5, Z being the thickness of the zone and t the depth of the thread. For different types and sizes of thread the value of t can be found in textbooks and standard specifications. If the zone thicknesses comply with this formula even screws down to M6 are not corroded by subsequent exposure to moisture.

It is preferred to perform the method according to the invention, especially in the case of bolts and screws up to M10, by providing the smooth blank with the chromised zone and by then rolling the full depth of the threads into the blank.

In the case of larger sized bolts and screws, especially above M12, the invention is best performed by pre-forming the threads on the blank by provisional rolling and by then providing the chromised zone, the full depth of the threads being rolled in subsequently. In this procedure the above specified rule relating to the differential displacement of material and the zone depth distribution must likewise be observed. The depth of the zone must be matched to the degree of deformation when rolling the final thread in order to ensure a minimum thickness of 0.05 mm. at the roots of the threads after the final roll has been performed. Since the required thickness of the zone which is to be created varies to some extent with the derisk of seizing despite the greater tightness of lit.

3 gree of deformation in the provisional and final rolling, a few tests should be made in each particular case for determining the necessary thickness, for instance by preforming a thread to a depth of 50% of the final depth, then producing, a zone of appropriate thickness in accordance with the above specified rule and by final rolling to produce the finished thread. The rule specified by the invention enables a person skilled in the art to achieve satisfactory results when applying the method to any se lected rolling conditions.

The afore-mentioned rule generally permits threads to be produced to given specifications, which will exhibit the above described favourable zone distribution. When compared with bolts and screws into which finished threads are rolled before they are provided with thicker zones, the production of correspondingly thicker zones according to the invention followed by rolling in the threads leads to a consolidation or the chromised surfaces and to the formation of threads or" ldgher precision. Moreover, rolled threads formed after the blanks have been chromised do not as readily seize. As known, it has been proposed to overcome the trouble of seizing by chromising nuts and bolts in which the clearances are greater than normal. However, in many instances increased clearances are not desirable. In the case of threads formed by rolling after the surfaces have been chrornised, the tolerances can be narrower or at least Within the conventional range, with the simultaneous elimination of the Moreover, above all, rolling the threads after chromising, that is to say, the reversal of the usual sequence of operations, produces chromised threads in which the chromised zones A at the delicate crests are stronger and better consolidated than those or" chronised threads produced in the conventional manner. e

What I claim is:

l. The method of producing chromised screws and bolts of a diameter exceeding 10 mm, with rolled threads, which comprises the steps of: first rolling a provisional thread in the bolt blank, then producing the chromised zone to a thickness equal to the depth of the thread-part to be finally rolled divided by 2.5, and thereafter finally rolling the thread to the full depth and so that the thickness of the chromised zone at the bottom of the finish-rolled thread will not be less than 0.05 mm.

2. The method of producing chromised screws and bolts of a diameter under 10 mm. with rolled threads, which comprises the steps of: difiusing chromium on a smooth blank to produce a chrornised zone thereon having a thickness equal to the depth of the thread to be rolled divided by 2.5, and then rolling the thread to the said depth.

Eited in the tile of this patent UNITED STATES PATENTS Von Forster Apr. 26, 1938 Schaurte May 27, 1941 OTHER REFERENCES Serial No. 394,370, Lickteig and Neuman (A.P.C.), published June 1, 1943.

Protective Coatings for Metals, by Burns and Bradley, second edition, published by Reinhold Publishing Corporation, 430 Park Avenue, New York, 1955. Pages 229-230 relied on.

Reterenees 

2.THE METHOD OF PRODUCING CHROMISED SCREWS AND BOLTS OF A DIAMETER UNDER 10 MM. WITH ROLLED THREADS, WHICH COMPRISES THE STEPS OF: DIFFUSING CHROMIUM ON A SMOOTH BLANK TO PRODUCE A CHROMISED ZONE THEREON HAVING A THICKNESS EQUAL TO THE DEPTH OF THE THREAD TO BE ROLLED DIVIDED BY 2.5, AND THEN ROLLING THE THREAD TO THE SAID DEPTH. 